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E39 (1997 - 2003) The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

My speed sensor went on me and the exact same problem happened. Replaced the sensor and no issues so far. My dads VW Jetta had the same problem as well. His alarm was going off at 2,3, sometimes even 4am until he replaced his sensor. Is this common in German Cars?

Just unplug the hood switch (on the front corner of the drivers side cabin filter) and I'll bet the problem goes away. If so you can replace the switch or just leave it unplugged. That's the source of most false alarm issues.

I have seen several threads for the alarm going off at random. My E39 530i started the same issue. I replaced the hood switch and it still went off at random. I unplugged the interior sensor, and it still went off at random. I removed fuse 48 from the trunk fuse panel, and even though siren would not sound, lights would still flash while alarm was going off at random. Come to find out that there is an updated hood switch, that originally came off the X5, with a longer plunger. When I had the alarm code read, it said the hood was opened 31 times. I picked it up from my local dealer for about $18 and no problems at all now. Hope this helps some of you.

I am having this nightmare issue today.
The alarm only goes off when I access the car (unlock via the key)

I disconnected the hood latch socket. Alarm still goes off.
I pulled out Fuse 48 in the trunk. This actually set off the alarm again!
I will now test the battery voltage, but I've had low voltage before and never had the alarm fritz! (12.6v off and 14v on)

I can lock it twice to disable interior sensor, but that's not my issue (the key unlock sets off the alarm)
Just in case, is it easy to unplug the interior sensor??

PS: I HATE car alarms. Is there a wire I can just cut?
This is only one step below your car being on fire.
I'd rather have my tranny die, honestly. Less drama and attention.

A new battery seems to have solved my car alarm issue!
I was locking and unlocking the car with glee (and silence)

As a postmortem, I did notice my door locks were occasionally acting "weak" over the last few weeks.
Sometimes, they'd hesitate when unlocking (weak battery?).
I also noticed I had to sometimes unlock again when they first try seemed to abort/stall.
Otherwise, there was no indication of a low battery.
The locking/unlocking now seems more "crisp", but I'm not sure if that's the placebo effect.

A new 49-DL was $118 out the door at AutoZone. Here is the DIY
Even if this isn't the real culprit, I don't mind dropping $100 on a service item. My Excide was about 2 years old, FWIW.